Directory of Mark Twain's maxims, quotations, and various opinions:

JOURNAL

At certain periods it becomes the dearest ambition of a man to keep a
faithful record of his performances in a book; and he dashes at this work
with an enthusiasm that imposes on him the notion that keeping a journal
is the veriest pastime in the world, and the pleasantest. But if he only
lives twenty-one days, he will find out that only those rare natures that
are made up of pluck, endurance, devotion to duty for duty's sake, and
invincible determination, may hope to venture upon so tremendous an enterprise
as the keeping of a journal and not sustain a shameful defeat.
- The Innocents Abroad

If you wish to inflict a heartless and malignant punishment upon a young
person, pledge him to keep a journal a year.
- The Innocents Abroad